This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(644). The second class of organs to be enumerated as entering into the composition of the lateral appendages are soft, fleshy, and subarti-culated processes called cirri (fig. 118,2, c, e); these are generally two in number, and belong one to the ventral and the other to the dorsal oar: their precise office is not well understood; but as in some of the segments, especially in the neighbourhood of the head, they assume a tentacular form, they have, with much probability, been regarded as instruments of touch.
(645). The setae (fig. 118,2, d) are, perhaps, the most efficient agents in progression. These are long and stiff hairs disposed in bundles and implanted into strong muscular sheaths. Each packet of setae can be retracted within the body to a certain extent, and again protruded by the action of the tubular supports from which they arise; and being capable of independent action, these organs must be looked upon as so many powerful fins, well calculated to propel the creature through the element it inhabits.
(646). The structure of the mouth in the Dorsibranchiate Annelidans is very peculiar. The first portion of the alimentary canal, or stomach, as it is most erroneously called by some writers, is muscular; and certainly, when seen in a dead Annelid, it might easily be taken for a digestive cavity. Nevertheless, during life, this part of the alimentary apparatus is destined to a widely-different office; for it is so constructed that, at the will of the animal, it can be completely everted, turned inside out, and, when thus protruded externally, it forms a very singular proboscis, used in seizing food, and frequently armed with powerful teeth of singular construction. The following figure (119, a), representing the head of one of these worms (Goniada a chevrons, Milne- Edwards), will give a good idea of this curious organ when fully displayed; and in fig. 119, B, the mechanism is exhibited by which its protrusion and retraction are accomplished. The whole apparatus is there seen to consist of two muscular cylinders, placed one within the other, but continuous at their upper margin (b); or, to use a familiar illustration, the proboscis may be compared to the finger of a glove partially inverted. It is obvious that, in this case, if the inner cylinder be drawn inwards - that is, into the mouth, - the whole structure becomes shortened, until at last it is entirely retracted into the oral cavity; whereas, on the contrary, if the outer tube is made to protrude, it expands at the expense of the inner one, which is gradually drawn forwards. The internal surface of this remarkable proboscis, moreover, is variously modified in its structure, so as to adapt it to the prehension of different kinds of prey. In Amplrinorne, for instance, the orifice of the mouth is a thick, fleshy, and callous circle (fig. 122, b, c, d), and the surface of the exserted proboscis (c, d) is covered with delicate transverse rugae, evidently so arranged as to give tenacity to its gripe. In Goniada it supports two distinct sets of horny teeth, provided for very different uses: one set, which is exposed when the proboscis is unrolled to a very small extent, consists of a series of linear horny plates (fig. 119, a, d), and probably forms a kind of file, or rather a scraper, wherewith the animal excavates the subterranean galleries in which it lives. The other set does not make its appearance till the proboscis is more completely expanded, and is evidently an instrument of prehension, formed by two horny hooks (fig. 119, b, a, b) placed upon an elevated ridge near the entrance of the oesophagus, so as to take a secure hold of any victim seized by this curious mouth.
Fig. 119. Mouth of Goniada clavigera.
Fig. 120. Mouth of Phyllodoce laminosa.
(647). In Phyllodoce laminosa the teeth form a circle of semicarti-laginous beads encompassing the extremity of the proboscis when that organ is pushed out to its full length (fig. 120, b), - an arrangement well adapted to hold and, perhaps, to crush their prey.
(648). But the most formidable jaws are met with in some of the Nereidiform species, as in Laodicea antennata, of which a figure is given above (fig. 118.) When the proboscis of one of these creatures is slightly everted, the extremities of three pairs of strong horny plates emerge from the mouth: of these, one pair terminates by forming a powerful hooked forceps, while the others present strong denticulated margins (fig. 121, a, a, b, c.) The nature of these teeth will be better* seen by a glance at b in the same figure, where they are represented, upon an enlarged scale, as they appear when detached from their connexions.
Fig. 121. Jaws of Laodicea antennata.
(649). The alimentary canal of the Dorsibranchiate Annelidans offers little which requires special notice. It invariably passes in a direct line from the termination of the proboscis to the anal extremity of the body. In the Nereidce it is provided with numerous lateral pouches, somewhat resembling those of the Leech. In Aphrodite these lateral caeca are very long, slender, and branched at their extremities, so that they have been thought by some to be secreting organs, representing the liver. In Arenicola we find, at the termination of the oesophagus (fig. 128, f), two large caecal appendages (e), of unknown office, while the rest of the tube(c) is entirely covered with minute saccali, the walls of which are decidedly glandular and secrete a fluid of a greenish-yellow colour.
(650). In the majority of the Annelids, observes Dr. Williams, the alimentary system constitutes a cylindrical tube, which bears a general resemblance of outline to the integumentary, - this latter forming, with respect to the former, an exterior concentric or embracing cylinder. These two cylinders are in no instance in agglutinated contact: a space intervenes, varying in capacity in different species, to designate which the term'peritoneal,' or splanchnic, may be used with perfect propriety. This space is occupied by a vital or organized fluid charged with corpuscles, which exhibit, under the microscope, characters distinctive of species. Independently of its physiological uses, this fluid enacts mechanical functions indispensable to the well-being of the animal: on it, as upon a pivot, the vermicular motions of the intestinal cylinder are performed.